This invention relates to the formation of an electrically conductive, freestanding microporous polymer sheet and, in particular, to such a sheet for use in the manufacture of energy storage and other suitable devices including supercapacitors, pseudocapacitors, electrochemical capacitors, double layer capacitors, electrochemical double layer capacitors, hybrid capacitors, asymmetric capacitors, and ultracapacitors.
The following background information is presented by way of example with reference to the manufacture of electrodes used in energy storage devices. Descriptions of the construction details of energy storage devices relevant to the present invention are set forth in A. Burke, Ultracapacitors: why, how, and where is the technology, J. Power Sources 91, (2000) pp. 37-50.
Ultracapacitors differ from batteries in that they provide higher power density, excellent reversibility, and very long cycle life. Exemplary charge-storage mechanisms of ultracapacitors include double layer capacitance and charge transfer pseudocapacitance. Double layer capacitance arises from the separation of charge at a solid-electrolyte interface, whereas pseudocapacitance involves reversible faradaic reactions occurring at a solid surface over a defined potential range.
Significant effort has been devoted to research focusing on the use of high surface area carbon powders as the electrochemically active material in ultracapacitors. While some of these powders have specific capacitance values in excess of 100 Farads/gram, their low densities provide a much lower volumetric capacitance values, which are of importance in ultracapacitor fabrication. Furthermore, the micropores ( less than 2 nm diameter) of activated carbons are often not accessible to the electrolyte in an ultracapacitor, resulting in no double layer formation and lower than expected capacitance. Carbon aerogels are a unique form of carbon derived from the sol-gel polymerization of organic monomers, such as resorcinol and formaldehyde, followed by pyrolysis at elevated temperature ( greater than 800xc2x0 C.). As discussed in Pekala et al., Structure and Performance of Carbon Aerogel Electrodes, Materials Research Society Symposium Proceedings 349, (1994) pp. 79-85, carbon aerogels can be synthesized over a wide range of densities with high surface areas (600-800 m2/g), a predominance of mesopores (2-50 nm), and low electrical resistivity. This microstructure provides high volumetric capacitance values for carbon aerogel monoliths and powders. As such, the incorporation of carbon aerogels into a free-standing, microporous polymer sheet is of great interest as a new method for the fabrication of ultracapacitor electrodes.
Many transition metal oxides and mixed metal oxides have also been investigated as electrochemically active materials for ultracapacitors where the principal charge-storage mechanism is pseudocapacitance. Certain forms of ruthenium oxide have specific capacitance values as high as 750 Farads/gram. Other metal oxides such as tantalum oxide, manganese dioxide, lead oxide, and nickel oxide are under investigation. In each case, the incorporation of these materials into a freestanding, microporous polymer sheet has not been contemplated for the fabrication of ultracapacitor electrodes.
Ultracapacitors can also be fabricated with one electrode being of a double layer material (e.g., activated carbon) while the other electrode is made from a pseudocapacitance material (e.g., ruthenium oxide). Such energy storage devices are referred to as hybrid or asymmetric capacitors.
Electrode preparation for many energy storage devices begins with the formation of a slurry containing an electrochemically active material in powder form, a fluoropolymer, and solvent. The slurry is coated onto a metal foil that acts as a current collector. The metal foil coated with the electrochemically active material is then passed through a drying oven to remove the solvent. The fluoropolymer acts as a binder that holds together the electrochemically active material and forms a porous electrode. Often the electrode is calendered to densify the electrochemically active material coated on the current collector by increasing the volume or packing fraction of the electrochemically active material and thereby reducing the porosity of the electrode. The current collector functions also as a carrier for the electrochemically active material and the binder because the combination of the two of them is of insufficient mechanical integrity to stand on its own as a freestanding, microporous polymer sheet. The electrode is then cut into ribbons for winding or stacking into a packaged energy storage device.
Fluoropolymers, such as polyvinylidene fluoride, have historically been used as polymer binders because of their electrochemical and chemical inactivity in relation to most polymer, gel, or liquid electrolytes. However, it is difficult, if not impossible, to produce freestanding porous electrodes utilizing fluoropolymers at traditional binder contents (2-10 wt. %) because their low molecular weights provide inadequate chain entanglement. Other binders such as EPDM rubber and various types of polyethylene can be used, but they also do not provide microporous sheets with freestanding properties. xe2x80x9cFreestandingxe2x80x9d refers to a sheet having sufficient mechanical properties that permit manipulation such as winding and unwinding in sheet form for use in an energy storage device assembly.
A special type of polyethylene, ultrahigh molecular weight polyethylene (UHMWPE), can be used to make a microporous sheet with freestanding properties at the binder contents specified above. The repeat unit of polyethylene is shown below:
(xe2x80x94CH2CH2xe2x80x94)x, 
where x represents the average number of repeat units in an individual polymer chain. In the case of polyethylene used in many film and molded part applications, x equals about 103-104 whereas for UHMWPE x equals about 105. This difference in the number of repeat units is responsible for the higher degree of chain entanglement and the unique properties of UHMWPE.
One such property is the ability of UHMWPE to resist material flow under its own weight when the UHMWPE is heated above its crystalline melting point. This phenomenon is a result of the long relaxation times required for individual chains to slip past one another. UHMWPE exhibits excellent chemical and abrasion resistance, and the hydrocarbon composition of UHMWPE has a much lower skeletal density (0.93 g/cc) than many of the fluoropolymers commonly used in electrode preparation. Such commonly used fluoropolymers include polyvinylidene fluoride (1.77 g/cc) and polytetrafluoroethylene (2.2 g/cc).
UHMWPE is commonly used as the polymer matrix or binder for separators used in lead-acid batteries. Such separators result from the extrusion, calendering, and extraction of mixtures containing UHMWPE, precipitated silica, and processing oil. The resultant separators have many advantages: high porosity (50-60%), a dentritic growth-inhibiting ultrafine pore size, low electrical resistance, good oxidation resistance, and sealability into a pocket configuration. These separators usually contain a silica to UHMWPE weight ratio from about 2.5 to about 3.5 or a corresponding volume fraction ratio in the range of 1.0 to 1.5. Such separators are designed to prevent electronic conduction (i.e., short circuits) between the anode and cathode while permitting ionic conduction via the electrolyte that fills the pores.
While UHMWPE is an integral part of separator technology, its use in the extrusion and extraction of free-standing, electrically conductive porous film electrodes has never been achieved. This invention addresses the desire to fabricate such film electrodes for use in energy storage and other electronic device applications.
An object of the present invention is, therefore, to provide an electrically conductive, freestanding microporous polymer sheet formed with a relatively high volume fraction of the electrically conductive matrix (composed of an electrochemically active powder and an electrically conductive agent, if required) to the polymer matrix and having sufficient mechanical properties for use as ultracapacitor electrodes. An electrochemically active powder is one that exhibits sufficient double-layer capacitance or pseudocapacitance for the purpose of this invention.
The present invention is a freestanding, microporous polymer sheet that is composed of a polymer matrix binding a material composition (i.e., the electrically conductive matrix) having electrical conductivity properties. The polymer matrix preferably includes UHMWPE, and the material composition preferably contains one of a carbonaceous material and a metal oxide, or a combination thereof. Exemplary carbonaceous materials include high surface area carbon ( greater than 250 m2/g), activated carbon, and carbon aerogel. Exemplary metal oxides include ruthenium oxide, tantalum oxide, manganese dioxide, nickel oxide, and lead oxide. The UHMWPE is of a molecular weight that provides sufficient molecular chain entanglement to form a sheet with freestanding characteristics, and the material composition powders have relatively high surface areas. Preferably, the polymer matrix of the microporous sheet does not exceed a volume fraction of about 0.20.
Multiple microporous sheets can be wound or stacked in a package filled with an electrolyte to function as electrodes in an energy storage device, such as a battery or an ultracapacitor. Metallic layers can be applied to the microporous sheets to function as current collectors in such devices.
In a first preferred embodiment of the invention, the freestanding, microporous polymer sheet is manufactured by combining UHMWPE, a material composition in powder form and having electrical conductivity properties, and a plasticizer (e.g., mineral oil). A mixture of UHMWPE and the material composition powder is blended with the plasticizer in sufficient quantity and extruded to form a homogeneous, cohesive mass. A blown film process or another traditional calendering method is used to shape the oil-filled sheets to their final thicknesses. In an extraction operation similar to that used for the production of lead acid battery separators, the oil is removed from the sheets. Metallic layers are then applied to the extracted sheets to form current collectors. A metallic layer can be one of a metal film formed by sputter deposition on, electroless deposition on, electrodeposition on, plasma spraying on, or roll coating of a metal slurry on the microporous sheet; or a porous or nonporous metal foil laminated to the microporous sheet. In some cases, sufficient metal powder can be incorporated in the polymer sheet such that a metallic layer as described above is not required.
In a second preferred embodiment of the invention, a polymer matrix, containing an UHMWPE in an amount and of a molecular weight sufficient to provide the necessary molecular chain entanglement to form a freestanding microporous sheet, binds a material composition having electrical conductivity properties. The resulting electrically conductive sheet is wound or stacked in a package, and the pores of the sheet are filled with an electrolyte and used as one of many electrodes in an energy storage device, for example, a battery, capacitor, supercapacitor, or fuel cell. One of the benefits of this polymer matrix is that it can be used to form, and potentially provide intimate contact between adjacent electrode and separator layers.
In a third preferred embodiment of the invention, multiple electrode and separator layers are coherently bonded to one another to form an ultracapacitor. One preferred method of coherently bonding the multiple layers involves simultaneously coextruding the layers through multiple extruders. A second preferred method involves laminating individual layers together. These processes promote an integral, coherent bond between adjacent electrode and separator layers and reduce the risk of delamination during extraction. These processes also provide intimate contact between the porous electrodes and the separator without collapsing porosity at adjacent layer interfaces. The resultant multiple layer ribbon with one or more current collectors is cut to size, and the pores are filled with electrolyte to produce an energy storage device.
Additional objects and advantages of this invention will be apparent from the following detailed description of preferred embodiments thereof which proceeds with reference to the accompanying drawings.